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Scientists have solved a long-standing ecological conundrum: why nitrogen-fixing plants only thrive in areas of the world that don't appear to need them.

The findings, which appear in today's issue of the journal Nature, will help in the development of more accurate climate change models say researchers.

Australian plant physiologist and co-author Dr Ying-Ping Wang, of CSIRO Marine and Atmospheric Research in Aspendale, says nitrogen-fixing plants have bacteria living on their roots that capture atmospheric nitrogen.

He says the bacteria pass nitrogen onto the plant in return for carbon-rich sugars and theoretically, these plants should be most common in parts of the world that have low amounts of nitrogen in the soil.

So scientists have been puzzled as to why such plants naturally thrive in nitrogen-rich tropical forests, but not in nitrogen-poor temperate forests, says Wang.